We now turn to a rather special form of Mapping, the CmpFrame. The Frames we have considered so far have been atomic, in the sense that they represent pre-defined elementary physical domains. A CmpFrame, however, is a compound Frame. In essence, it is a structure for containing other Frames and its purpose is to allow those Frames to work together in various combinations while appearing as a single Object. A CmpFrame's behaviour is therefore not pre-defined, but is determined by the other Frames it contains (its ``component'' Frames).
As with compound Mappings, compound Frames can be nested within each
other, forming arbitrarily complex Frames.
AST A Library for Handling World Coordinate Systems in Astronomy